The artist I'm adding/editing performs under a subgenre that is not available on Viberate. How should I determine genre/subgenres in such cases?

Genres are very important. Not only that they give us information about the artist's music style but they also position the artist to the corresponding genre and subgenre charts. If a certain subgenre is not available choose the one or the combination of subgenres that is closest to the artist's style.  Listen to a few tracks before you decide on certain subgenres. 

Please note that the first selected subgenre determines artists' main genre.